This invention relates to a combinatorial counting method for use in a case where the individual or unit weight of an article varies slightly from one article to the next, as in the case of nuts and bolts and small packaging cups for milk or other liquids, especially where the individual weight of supplied articles varies gradually with time. More particularly, the invention relates to combinatorial counting method wherein it is possible to lengthen the time period during which the total weight of articles discharged from a weighing system is substantially constant.
A combinatorial counting apparatus operates by weighing articles through use of a plurality of weighing machines supplied with the articles, converting the weight value measured by each weighing machine into the number of articles supplied to that machine, and computing combinations of the article numbers provided by each weighing machine to obtain a set number of the articles. In the combinatorial counting apparatus of the type described, the weight-to-number conversion is performed by dividing the weight measured by each weighing machine by the individual weight (namely the weight per article, referred to as the "unit" weight hereafter of the articles contained in that weighing machine. When an error develops in the unit weight of the article, however, the number of articles computed to be contained in each weighing machine is subjected to an error, with the result that the combined number of articles computed by combinatorially adding the articles in each of the weighing machines is also subjected to an error. The articles therefore cannot be counted accurately. Another problem is that the greater the number of articles to be counted by a single weighing machine, the greater will be the error in the computed number of articles. In order to accurately compute the number of a multiplicity of articles, therefore, it is required to first obtain a correct measure of the unit weight of each article. While this may be acceptable where the unit weights are constant, however, in actual practice the units weights are not constant because some articles are inevitably lighter or heavier than others. In such case it is essentially meaningless to find unit weights even if each article is weighed correctly. It is better practice to weigh a multiplicity of articles and divide their total weight by the number of the articles to determine the average unit weight. The more articles to be weighed, the more correctly the average unit weight can be obtained.
The present applicant has previously proposed, in Japanese Patent Application No. 55-133908, a counting method which includes the steps of weighing a multiplicity of articles with n-number of weighing machines, dividing the total weight measured by each weighing machine by the unit weight of the article to find the number of articles contained in each of the weighing machines, computing various combinations of numbers of articles to determine the number of articles thus combined, and finding one of the combinations having the number of articles equal or closest to a preset number. Subsequent steps include computing a new unit weight based on the total weight of articles contained in any single weighing machine or plurality thereof and the total number of articles contained in said single or plurality of weighing machines, relying upon the new unit weight in the next counting operation for computing the number of articles.
Although the foregoing method of counting articles is successful in diminishing counting errors, a problem remains in a case where the unit weight of the articles fed into the weighing machines varies with time, for this will cause the total weight of the articles discharged from the weighing machines to vary with time as well, so that the total weight of discharged articles will not be constant. By way of example, let us assume that the unit weight of the articles fed into the weighing machines increases with time. In such case, the total weight of the discharged articles will vary as indicated by the marks (O) in FIG. 1. This curve clearly shows the disadvantage encountered in the method previously proposed, namely that there is a shortening of the time interval during which the total weight of the discharged articles is constant.